1. Field of the Invention
The present invention relates to a washing device and its method for an impression cylinder jacket in a sheet-fed offset two-sided (perfecting) printing press.
2. Description of the Related Art
The sheet-fed offset two-sided printing press with multicolor units is divided into two categories: one being an arrangement where the sheet is reversed during the course of its travel through plural printing units arranged only on one side of the sheet passage (either the top or the bottom) in order to be printed on both sides of the sheet, and the other being an arrangement where both sides of the sheet is alternately printed without reversing it by plural printing units arranged on both sides of the sheet passage.
In either case, while the ink on the already printed side of the sheet is not set yet and is still in contact with the impression cylinder, the other side of the sheet is printed as the sheet is pressed between a blanket cylinder and the impression cylinder. As a result, the ink on the already printed side of the sheet is transferred to the impression cylinder. Since the ink transferred to the impression cylinder is transferred back to the sheets to be fed thereafter, it causes staining in the printing process, thus making multicolor printing impossible under such a condition.
In order to solve this problem, technologies such as the one disclosed by Unexamined Publication No. JP-A-8-12151 are proposed. The technology disclosed by said publication intends to prevent the ink on paper immediately after printing from being transferred to the jacket installed on the impression cylinder by means of installing a jacket having smooth concave-convex profile with a surface roughness of Rmax of 20-40 μm formed by coating a metallic plate with porous ceramics by means of thermal spray, and then coating on top of it with a low surface energy resin of the silicone group resin, etc. (herein after called “separating compounds”). Thanks to this technology, it has become possible to conduct two-sided printing with multicolor on paper such as art paper or coated paper which does not absorb ink too well.
However, even when this jacket is used, it becomes necessary to wash the jacket once every 60,000 sheets of printing when the jacket is new, and every 10,000 sheets as the separating compounds in the convex areas of the ceramics coating layer (hereinafter called “ceramic convexes”) on the jacket surface wears out.
If the jacket is to be manually washed, it requires the workers to work with unstable postures in very narrow space, so that there are problems in terms of safety and work efficiency. Consequently, various automatic washing devices for an impression cylinder jacket have been proposed and are used in order to solve the problems related to washing jacket by hand.
Let us now describe various types of conventional automatic washing devices for an impression cylinder jacket and their problems.
(1) First type of washing device with nonwoven fabric cloth impregnated with a washing liquid directly pressed against the impression cylinder
FIG. 1 shows the first type of conventional washing device for an impression cylinder jacket. In the device shown in FIG. 1, while the machine is operated with a plate cylinder 121, a blanket cylinder 122, and an impression cylinder 123 rotating at a slow speed and all cylinders 121-123 are maintaining some distances from each other, nonwoven fabric cloth 131b impregnated with a washing liquid is directly pressed against impression cylinder 123, on which the jacket is mounted. This causes the ink on the jacket to be softened and the softened ink is then wiped out by the nonwoven fabric cloth. Also, blanket cylinder 122 is washed in a similar manner as nonwoven fabric cloth 131a is directly pressed against blanket cylinder 122. This type of washing device is divided into two types, one in which the washing liquid is sprayed from a nozzle on the dry nonwoven fabric cloth and the other in which the nonwoven fabric cloth is impregnated with a washing solvent beforehand.
However, the first type of washing device has a problem that not only the separating compounds in the convex ceramic areas but also the separating compounds in the concave areas on the jacket surface get worn out as the coarse nonwoven fabric cloth rubs the surface of the impression cylinder jacket strongly. The separating compounds is the key in preventing the ink from attaching to the jacket mounted on the impression cylinder. Therefore, if the separating compounds disappears as a result of the abrasion, the jacket can become easily stainable, thus causing staining on the printed matters, and causing a need for the jacket's replacement. In case of washing the jacket by hand to clean it, the life time of the jacket's replacement is typically required once in every 20 to 30 million printings, while the same cycle time becomes as short as once in 15 million printings, or approximately one half of the cycle time of the manual washing, when the first type of automatic washing device is used where the nonwoven fabric cloth is pressed directly against the impression cylinder.
When the impression cylinder rotates with the nonwoven fabric cloth pressed against the impression cylinder, the fibers of the nonwoven fabric cloth get tangled with the ceramic convexes on the surface of the jacket mounted on the impression cylinder and remain fibers on the jacket even after the washing process. These fibers can appear as white dots defects on the printed surface in the next printing job and caused the problem of printing defects.
Furthermore, the first type of washing device requires an expensive capital investment for the equipment since a washing device having a complicated mechanism with nonwoven fabric cloth needs to be installed into two very narrow spaces among blanket cylinder 122 and impression cylinder 123. Moreover, its running cost is quite high as the nonwoven fabric cloth needs to be frequently replaced.
(2) Second type of washing device having a brush roller pressed against and rotated with the impression cylinder in which the washing liquid is fed to the roller
FIG. 2 shows the second type of conventional washing device for an impression cylinder jacket. In the device shown in FIG. 2, while the machine is operated with a plate cylinder 121, a blanket cylinder 122, and an impression cylinder 123 rotating at a slow speed and each of cylinders 121-123 is maintaining some distances from each other, a brush roller 141b, to which a washing liquid is fed, is directly pressed against impression cylinder 123 having the jacket. This makes it possible to wash down the ink on the jacket. Two different kinds of washing liquids can be fed through two nozzles 142b and 143b. Blanket cylinder 122 can also be washed similarly by having brush roller 141a, to which two different kinds of washing liquids are fed through two nozzles 142a and 143a, directly pressed against blanket cylinder 122. The used washing liquids can be either recycled or wasted.
However, the second type of washing device can not only remove the separating compounds on the ceramic convex areas on the jacket surface but also cause wears of the separating compounds in the concave areas, so that it shortens the life time of the jacket substantially similar to the abovementioned first type, since the brush is directly contacting the jacket surface while it is rotating.
Furthermore, similar to the first type of washing device, it requires an expensive capital investment for the equipment since a washing device having a complicated mechanism with a brush roller needs to be installed into two very narrow spaces among blanket cylinder 122 and impression cylinder 123. Moreover, it requires an expensive investment for the detergent recycling device if it is used, and if the washing liquid is not recycled, it requires a large scale waste liquid process.
(3) Third type of washing device having a cleaning unit that can be selectively contacted to the blanket cylinder that is opposing the impression cylinder, in which the blanket cylinder rotates for a definite period of time while it is contacting the impression cylinder with the cleaning unit pressed against the blanket cylinder
FIG. 3 shows the third type of conventional washing device for an impression cylinder jacket. This third type of washing device is disclosed in said Unexamined Publication No. JP-A-8-12151, and is a washing device for washing a jacket formed a hybrid coating layer consisting of a thermal sprayed ceramic layer and a separating compounds layer.
In the device shown in FIG. 3, the machine (each of cylinders 121-123) is rotated at a slow speed while only plate cylinder 121 is separated from other cylinders with blanket cylinder 122 and impression cylinder 123 are contacted with each other, and nonwoven fabric cloth 131a impregnated with a washing liquid is pressed against blanket cylinder 122. Therefore, the ink on the jacket mounted on impression cylinder 123 softens as a result of a small amount of washing liquid transfered from blanket cylinder 122 while the blanket cylinder 122 and impression cylinder 123 make contact rotations, and the ink is transferred back to the blanket cylinder very easily because the jacket surface is coated with the separating compounds. Thus, the jacket mounted on impression cylinder 123 can be completely washed only by means of the washing mechanism having nonwoven fabric cloth 131a mounted on blanket cylinder 122.
Moreover, although blanket cylinder 122 and impression cylinder 123 are pressed to each other with a strong pressure, no abrasion occurs on the separating compounds on the surface of the jacket mounted on the impression cylinder because there is not any slip between the two cylinders at all. Further, needless to say, the nonwoven fabric cloth is not contacting the jacket directly so that there does not happen any problem about residual of the fibers from the nonwoven fabric cloth left on the jacket; it is indeed an excellent washing device.
This third type of washing device can be retrofitted on an existing press with a minimum capital investment as it requires almost no modification, requiring simply an addition of an electrical sequence, if blanket cylinder 122 can be stopped at a position when it contacts with impression cylinder 123, as it uses a method of causing blanket cylinder 122 to contact with impression cylinder 123 first and then contact with plate cylinder 121 as a means of causing plate cylinder 121 to contact with blanket cylinder 122, and blanket cylinder 122 with impression cylinder 123 (cylinder engagement mechanism system of the printing press).
However, the most popular cylinder engagement mechanism system at the moment is the method of causing blanket cylinder 122 to contact with plate cylinder 121 first, and then to contact with impression cylinder 123. If blanket cylinder 122 is washed while blanket cylinder 122 is in contact with impression cylinder 123 in such a cylinder engagement mechanism system, the jacket mounted on impression cylinder 123 gets washed of course, but the washing liquid transfers to plate cylinder 121 via blanket cylinder 122. If the washing liquid puts on plate cylinder 121, a plate installed on plate cylinder 121 can become sensitized, which causes staining, which is considered a defective printing. Therefore, this third type of washing device presents a problem that it cannot be adopted in current mainstream of printing presses.
On the other hand, if the third type of washing device is to be applied as is to the current mainstream of printing presses, it is necessary to modify the mechanism to allow blanket cylinder 122 to contact only with impression cylinder 123 in order to prevent the washing liquid from transferring to plate cylinder 121. This requires a substantial design change in the mechanism and the capital investment for the modification can be quite large.